accel/ivpu: Document why full JSM message size is always used

accel/ivpu: Document why full JSM message size is always used

Firmware expects IPC messages to always carry the full fixed
sizeof(struct vpu_jsm_msg) size. Sending the full struct also
ensures unused fields are zeroed, which maintains compatibility
when existing commands are extended with new fields in the future.

Replace the misleading TODO comment with an explanation of the
actual intent.

Reviewed-by: Dawid Osuchowski <dawid.osuchowski@linux.intel.com>
Signed-off-by: Karol Wachowski <karol.wachowski@linux.intel.com>
Link: https://patch.msgid.link/20260522093209.1169716-1-karol.wachowski@linux.intel.com